Personne : Vanasse, Anne
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Université Laval. Département de phytologie
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- PublicationAccès libreThreshold dynamics in plant succession after tree planting in agricultural riparian zones(British Ecological Society, 2016-04-26) Vanasse, Anne; Poulin, Monique; González, Eduardo; Andersen, Roxane; Bourgeois, Bérenger1. Trajectories of plant communities can be described by different models of plant succession. While a clementsian (gradual continuum model) or gleasonian approach (relay floristics model) have traditionally been used to inform restoration outcomes, alternative succession models developed recently may better represent restoration trajectories. The threshold dynamics succession model, which predicts an abrupt species turnover after an environmental threshold is crossed, has never been used in a restoration context. This model might, however, better describe shifts in plant competitive ranking and facilitation interactions during species turnover. 2. Fifty-three riparian zones, planted with trees 3 to 17 years prior to sampling, and 14 natural riparian forests were studied in two agricultural watersheds of south-eastern Québec (Canada). The cover of vegetation strata was assessed at the site-scale, and the cover of plant species was estimated in a total of 784 1-m2 plots. Canopy cover was measured stereoscopically for each plot. 3. As revealed by Principal Response Curves (PRC) and broken stick models, herbaceous species composition was stable during the first 12–13 years after tree planting, but then abruptly shifted. This two-step pattern in species turnover followed the increase of canopy cover after tree planting. Once canopy cover passed a threshold of ca 40%, plant succession started and led to the re-establishment of forest communities 17 years after planting. 4. Following herbaceous species turnover, the cover of ecological groups changed significantly toward covers of natural riparian forests: shade tolerant species generally increased while light-demanding and non-native species decreased. Vegetation structure was also significantly affected by tree planting: tree and shrub cover increased while monocot cover decreased. 5. Synthesis and applications. Tree planting efficiently restored herbaceous forest communities in riparian zones by inducing a species turnover mediated by light availability corresponding to threshold dynamics model in plant succession. Fostering and monitoring canopy closure in tree-planted riparian zones should improve restoration success and the design of alternative strategies. The innovative statistical approach of this study aiming to identify succession patterns and their associated theoretical models can guide future restoration in any type of ecosystem around the world to bridge the gap between science and management.
- PublicationAccès libreEffects of competition, shade and soil conditions on the recolonization of three forest herbs in tree-planted riparian zones(Opulus Press, 2016-07-25) Vanasse, Anne; Poulin, Monique; Bourgeois, BérengerQuestions: In mesic forests, ecological filters due to past agricultural land-use reduce forest herbs recolonization. Is the recruitment of such species also limited in tree-planted riparian zones, by local filters such as competition, shade level and soil conditions? Location: Two agricultural watersheds, southeastern Québec, Canada. Methods: Three herbs characteristic of natural riparian forests were selected for this study: one graminoid, Glyceria striata, and two ferns, Matteuccia struthiopteris and Onoclea sensibilis. Effects of shade level (75% versus 50%) and soil type (forest versus agricultural soil) on seedling emergence were evaluated in a seed-sowing greenhouse experiment. In a two-year transplant field experiment, seedling and sporophyte establishment was monitored in five natural riparian forests and five tree-planted post-agricultural riparian zones on microsites with understory vegetation kept intact or cleared and on forest or agricultural soils. Using a priori contrasts, we assessed the influence of habitat type (natural riparian forests or tree-planted riparian zones), competition and post-agricultural soil type on transplant survival and growth. Results: Seedling emergence tended to be higher on forest soils for G. striata while sporophyte emergence increased under 75% shade for M. struthiopteris. Transplanted seedlings and sporophytes of the three species survived and grew as well in tree-planted riparian zones as in natural riparian forests. In tree-planted riparian zones however, competing understory vegetation reduced the survival and growth of G. striata and agricultural soil reduced the growth of M. struthiopteris. For O. sensibilis, only sporophyte survival was reduced by competition in tree-planted riparian zones. Conclusions: Planting trees in post-agricultural riparian zones fosters an establishment of forest herbs similar than those observed in natural riparian forests. Additional environmental filters specific to tree-planted riparian zones however offset the positive influence of trees and limit the recolonization of the three studied species. Considering the partial restoration success of establishment niches by tree planting, controlling spontaneous vegetation after tree planting is advised when conceivable and cost-effective to promote the recolonization of environmentally-limited forest herbs. Long-term transplant experiments should be more largely conducted to identify the ecological filters that reduce plant recolonization, and thereby design the most effective restoration strategies.
- PublicationAccès libreSpatial processes structuring riparian plant communities in agroecosystems : implications for restoration(Ecological Society of America, 2016-09-12) Vanasse, Anne; Poulin, Monique; González, Eduardo; Aubin, Isabelle; Bourgeois, BérengerThe disruption of hydrological connectivity by human activities such as flood regulation or land-use changes strongly impacts riparian plant communities. However, landscape scale processes have generally been neglected in riparian restoration projects as opposed to local conditions, even though such processes might largely influence community recovery. We surveyed plant composition of field edges and riverbanks in 51 riparian zones restored by tree planting (565 1-m2 plots) within two agricultural watersheds in southeastern Québec (Canada). Once the effects of environmental variables (hydrology, soil, agriculture, landscape, restoration) were partialled out, three models of spatial autocorrelation based on Moran's Eigenvector Maps and Asymmetric Eigenvector Maps were compared to quantify the pathways and direction of the spatial processes structuring riparian communities. The ecological mechanisms underlying predominant spatial processes were then assessed by regression trees linking species response to spatial gradients to seed and morphological traits. The structure of riparian communities was predominantly related to unidirectional spatial gradients from upstream to downstream along watercourses, which contributed more to species composition than bidirectional gradients along watercourses or overland. Plant traits selected by regression trees explained 22% of species response to unidirectional upstream-downstream gradients in field edges, and 24% in riverbanks, and predominantly corresponded to seed traits rather than morphological traits of the adult plants. Our study showed that even in agriculturally open landscapes, water flow remains a major force structuring spatially riparian plant communities by filtering species according to their seed traits, thereby suggesting long-distance dispersal as a predominant process. Preserving hydrological connectivity at the watershed-scale and restoring riparian plant communities from upstream to downstream should be encouraged to improve the ecological integrity of rivers running through agricultural landscapes.
- PublicationAccès libreEstablishment success of trees planted in riparian buffer zones along an agricultural intensification gradient(Elsevier Science, 2016-02-12) Bourgeois, Bérenger; Vanasse, Anne; Poulin, Monique; Rivest, D.Although riparian zones provide numerous ecological services, they have been widely degraded by agricultural intensification. To recover water quality and restore other critical services, tree planting has been implemented in agricultural riparian buffer zones worldwide. However, intensive agricultural practices adjacent to tree plantations are likely to impede their establishment. In this study, we assessed the survival and size of trees planted in riparian buffer zones along a gradient of agriculture intensification. We studied 68 riparian buffer zones in two agricultural watersheds of southeastern Québec (Canada) where trees had been planted 3–17 years prior to sampling. Tree survival and size (height, diameter and crown width) were measured and related to agricultural intensification, quantified as the frequency of annual crops in the agricultural field adjacent to riparian zones during the seven years prior to sampling. Tree survival decreased by 25% with increasing frequency of annual crops (P < 0.0001; R2 = 35%), independently of the planting year. Aside from the influence of tree age, tree size varied with the frequency of annual crops but only for three of the six most frequently planted tree species (P = 0.0007; R2 = 46%). These three species (Fraxinus pennsylvanica Marsh., Quercus macrocarpa Michx. and Picea glauca (Moench) Voss) showed reduced size with higher cultivation frequency of annual crops, whereas the other three species (Acer saccharum Marsh., Larix laricina (Du Roi) K. Koch and Quercus rubra L.) were more tolerant to agricultural intensification. While tree planting is carried out in riparian buffer zones to mitigate the environmental impacts of agricultural practices, agricultural intensification in turns impedes the establishment success of trees. To increase the environmental benefits provided by agroforestry projects, tree planting in riparian buffer zones should focus on species that tolerate agricultural intensive practices. Additionally, more frequent inclusion of hay meadows in the crop rotation of fields adjacent to riparian buffer zones may be beneficial to the establishment success of planted trees.